Plasmodium vivax is the most common malaria parasite affecting humans, comprising 56% of all human malaria cases reported in 1992, excluding Africa. In India, P. vivax is the primary cause of malaria. It is the predominant species found in temperate climates, but also occurs in large areas of the tropics, excluding Africa. Because the majority of Africans lack the Duffy blood group antigens, they are not susceptible to P. vivax. In Africa, P. ovale replaces P. vivax, occurring commonly only in areas populated by Duffy antigen negative individuals who are not susceptible to P. vivax. P. vivax and P. ovale are classified as benign tertian human malarias and share many similarities, including ability to infect nonhuman primates. Although the disease caused by P. vivax is seldom the sole cause of fatalities, it causes a severe, acute illness and, unlike P. falciparum, results in relapsing episodes of disease months or years after the original infection. Successful therapy requires clearance of both the blood and liver stages. Chloroquine-resistant P. vivax has been recently confirmed in Southeast Asia. A serious impediment to drug and vaccine development has been the inability to culture the parasites, which preferentially invade reticulocytes.
There has not previously been a standardized process for long-term cultivation of P. vivax in vitro. Mons, et al. (Parasitol. 66: 183-188 (1988)) disclosed a method for culture in infected monkey blood in a shaker culture for 5-6 cycles by adding reticulocytes obtained from bleeding monkeys that were treated with the hemolytic drug phenylhydrazine hydrochloride. Lanners (Parasit Res. 78:699-701)) reported maintaining parasites up to 20 days in a flow vessel in which human reticulocyte fractions (Percoll Renograffin gradient preparations) were added irregularly (at days 4, 9 and 13) to parasites originating from monkeys. The data indicates lack of continued recycling of parasites, since only one unhealthy trophozoite was seen at 20 days. Both procedures used RPMI 1640 medium supplemented with either 15% or 20% human serum. Neither procedure used alternate static and agitation phases during culturing. Both of the articles report that monkey parasitized cells became extremely fragile and ruptured before differentiation to the mature schizont stage.